Bump PHP and PHPUnit versions

Bumped PHP version to 7.3.0 and PHPUnit version to 9.5.* to meet
Moodle requirements.
Apart from that, for now, the sabberworm/php-css-parser version
has been fixed to 8.1.* because 8.2 onwards has a bug with
comments (raised by a couple of PHPUnit tests).

Author: sarjona

composer.json

@@ -6,11 +6,11 @@
     "license": "MIT",
     "minimum-stability": "dev",
     "require": {
-        "php": ">=5.4",
-        "sabberworm/php-css-parser": "*"
+        "php": ">=7.3.0",
+        "sabberworm/php-css-parser": "8.1.*"
     },
     "require-dev": {
-        "phpunit/phpunit": "*"
+        "phpunit/phpunit": "9.5.*"
     },
     "autoload": {
         "psr-4": {